1. Field of the Invention
This invention relates to methods for prophylactically and affirmatively treating various bodily states that respond beneficially to increases in extracellular levels of adenosine by providing patients with purine nucleosides, purine nucleotides, and derivatives, intermediates and analogs thereof. The invention also relates to the stabilization of mast cells with such compounds by suppression of mast cell activation.
2. Description of Related Art
Adenosine, 9-.beta.-D-ribofuranosyladenine (the nucleoside of the purine adenine), belongs to the class of biochemicals termed purine nucleosides and is a key biochemical cell regulatory molecule, as described by Fox and Kelly in the Annual Reviews of Biochemistry, Vol. 47, p. 635, 1978. It interacts with a wide variety of cell types and is responsible for a myriad of biological effects. For instance, adenosine is a potent vasodilator, an inhibitor of immune cell function, and can at certain levels enhance activation of mast cells, is an inhibitor of granulocyte oxygen-free radial production, is anti-arrhythmic, and is an inhibitory neurotransmitter. Considering its broad spectrum of biological activity, considerable effort has been aimed at establishing practical therapeutic uses for adenosine and its analogs.
Since adenosine is thought to act at the level of the cell plasma membrane by binding to receptors anchored in the membrane, past work has included attempts to increase extracellular levels of adenosine by administration of it into the blood stream. Unfortunately, adenosine is toxic at concentrations that have to be administered to a patient to maintain an efficacious extracellular therapeutic level, and the administration of adenosine alone is therefore of limited therapeutic use. Further, adenosine receptors are subject to negative feedback control following exposure to adenosine, including down-regulation of the receptors.
Other ways of achieving the effect of a high local extracellular level of adenosine exist and have also been studied. They include: a) interference with the uptake of adenosine with reagents that specifically block adenosine transport, as described by Paterson et al., in the Annals of the New York Academy of Sciences, Vol. 255, p. 402 (1975); b) prevention of the degradation of adenosine, as described by Carson and Seegmiller in The Journal of Clinical Investigation, Vol. 57, p. 274 (1976); and c) the use of analogs of adenosine constructed to bind to adenosine cell plasma membrane receptors.
There are a large repertoire of chemicals that can inhibit the cellular uptake of adenosine. Some do so specifically and are essentially competitive inhibitors of adenosine uptake, and others inhibit nonspecifically. P-Nitrobenzylthioguanosine appears to be a competitive inhibitor, while dipyridamole and a variety of other chemicals, including colchicine, phenethylalcohol and papaverine inhibit uptake nonspecifically.
Extracellular levels of adenosine can be increased by the use of chemicals that inhibit enzymatic degradation of adenosine. Previous work has focused on identifying inhibitors of adenosine deaminase, which participates in the conversion of adenosine to inosine. Adenosine deaminase activity is inhibited by coformycin, 2'-deoxycoformycin, and erythro 9-(2-hydroxy-3-nonyl) adenine hydrochloride.
A number of adenosine receptor agonists and antagonists have been generated having structural modifications in the purine ring, alterations in substituent groups attached to the purine ring, and modifications or alterations in the site of attachment of the carbohydrate moiety. Halogenated adenosine derivatives appear to have been the most promising as agonist or antagonist and, as described by Wolff et al. in the Journal of Biological Chemistry, Vol. 252, p. 681, 1977, exert biological effects in experimental systems similar to those caused by adenosine.
Although all three techniques discussed above may have advantages over the use of adenosine alone, they have several disadvantages, the major disadvantages being that they rely on chemicals that have adverse therapeutic side effects, primarily due to the fact that they must be administered in doses that are toxic, and that they affect nonselectively most cell types. As described in Purine Metabolism in Man, (eds. De Baryn, Simmonds and Muller), Plenum Press, New York, 1984, most cells in the body carry receptors for adenosine. Consequently, the use of techniques that increase adenosine levels generally throughout the body can cause unwanted, dramatic changes in normal cellular physiology.
With respect to post ischemic myocardial tissue and adenosine, it is stated in Swain, J. L., J. J. Hines, R. L. Sabina, and E. W. Holmes, Circulation Research 51:102-105 (1982), and in Holmes et al., U.S. Pat. No. 4,575,498 (issued Mar. 11, 1986), that adenosine concentration and blood flow are not altered in ischemic canine hearts exposed to the purine nucleoside 5-amino-4-imidazolecarboxamide riboside (AICA riboside). They also state that depletion of purine nucleotide pools, especially adenosine triphosphate (ATP), has been postulated to play a role in such dysfunction following, e.g., an ischemic event, and claim to have demonstrated an enhanced nucleotide synthesis and concomitant repletion of ATP pools by treating post-ischemic myocardium with the purine analog AICA riboside, stating that repletion of ATP pools should, in theory, enable the amelioration of tissue damage.
Several other groups of investigators, however, have published studies in which they were unable to demonstrate an enhanced repletion of ATP pools in ischemic tissue by the method of Swain et al., supra. Mentzer, R. M., Ely, S. W., Lasley, R. D., Lee, B. K. and Berne, R. M., Fed. Proc. 43:903 (1984); Mitsos, S. E., S. R. Jolly and B. R. Lucchesi, Pharmacology 31:121-131 (1985); Hoffmeister, H. M., Nienaber, C., Mauser, M. and Schaper, W. E., Basic Research in Cardiology 80:445-458 (1985); Mauser, M., H. M. Hoffmeister, C. Nienaber, and W. E. Schaper, Circul. Res. 56:220-230 (1985). In fact, Hoffmeister et al. demonstrate that ATP repletion by another mechanism does not improve cardiac dysfunction. Even Holmes and Swain have documented that AICA riboside does not effectively reach ATP because of an inhibition of the conversion of inosine monophosphate (IMP) to adenosine monophosphate (AMP). Sabina, R. L., Kernstine, K. H., Boyd, R. L., Holmes, E W. and Swain, J. L., J. Biol. Chem. 257:10178 (1982}; Amidon, T. M., Brazzamano, S., Swain, J. L., Circ. Suppli. 72:357 (1985); Swain, J. L., Hines, J. J., Sabina, R. L., Harburg, O. L. and Holmes, E. W., J. Clin. Invest. 74:1422-1427 (1984). Amidon et al., supra, state that "These results indicate that adenylosuccinate synthetase and/or lyase activities are limiting in isolated hearts and suggest that interventions designed to bypass IMP in AN (Adenine Nucleotide) synthesis might be more advantageous for increasing AN pool size." Swain et al., supra (J. Biol. Chem.), also demonstrated that AICA riboside does not alter ATP levels in non-ischemic myocardium.
While Mitsos et al., supra, claimed that their study demonstrated that AICA riboside infused intracoronary in high doses protected globally ischemic hearts from the mechanical dysfunction associated with an ischemic insult, Hoffmeister et al., Basic Res. Cardiol. 80:445-458 (1985), showed that on producing a reversible ischemia in dogs by coronary artery occlusion, AICA riboside application did not improve postischemic function and, in fact, worsened it. Swain et al., supra (J. Clin. Invest.) confirms the detrimental effects of high doses of AICA riboside on muscle contractility. Thus, the proposal that the administration of AICA riboside would be of benefit to patients after an ischemic event for repletion of ATP pools does not appear to be valid.
It will be appreciated from the foregoing discussion that a technique that would increase extracellular levels of adenosine or adenosine analogs at specific times during a pathologic event, that would increase these compounds without complex side effects, and which would permit increased adenosine levels to be selectively targeted to cells that would benefit most from them would be of considerable therapeutic use. By way of example, such a technique would be especially useful in the prevention of, or response during, an ischemic event, such as heart attack or stroke, or other event involving an undesired, restricted or decreased blood flow, such as atherosclerosis, for adenosine is a vasodilator and prevents the production of superoxide radicals. Such a technique would also be useful in the prophylactic or affirmative treatment of pathologic states involving increased cellular excitation, such as (1) seizures or epilepsy, (2) arrhythmias, and (3) inflammation due to, for example, arthritis, autoimmune disease, Adult Respiratory Distress Syndrome (ARDS), and granulocyte activation by complement from blood contact with artificial membranes as occurs during dialysis or with heart-lung machines. It would further be useful in the treatment of patients who might have chronic low adenosine such as those suffering from autism, cerebral palsy, insomnia and other neuropsychiatric symptoms, including schizophrenia. The compounds useful in the invention, which include AICA riboside, may be used to accomplish these ends.
Another area of medical importance is the treatment of allergic diseases, which can be accomplished by either preventing mast cells from activating, or by interfering with the mediators of allergic responses which are secreted by mast cells. Mast cell activation can be down-regulated by immunotherapy (allergy shots) or by mast cell stabilizers such as cromalyn sodium, corticosteroids and aminophylline. There are also therapeutic agents which interfere with the products of mast cells such as anti-histamines and adrenergic agents. The mechanism of action of mast cell stabilization is not clearly understood. In the case of aminophylline, it is possible that it acts as an adenosine receptor antagonist. However, agents such as cromalyn sodium and the corticosteroids are not as well understood.
It will be appreciated, therefore, that effective allergy treatment with compounds which will not show any of the side effects of the above-noted compounds, such as drowsiness in the case of the anti-histamines, agitation in the case of adrenergic agents, and Cushing disease symptoms in the case of the corticosteroids, would be of great significance and utility. In contrast to compounds useful in the invention, such as AICA riboside and ribavirin, none of the three known mast cell stabilizers are known or believed to be metabolized in the cell to purine nucleoside triphosphates or purine nucleoside monophosphates.